1. Field of the Invention
The present invention relates to semiconductor memory devices, and more particularly to a method and an apparatus for programming a virtual ground EPROM array cell without disturbing adjacent cells.
2. Related Art
The continuing demand for higher speed and lower cost semiconductor memories has led to the development of the virtual ground memory design for reprogrammable nonvolatile memories. A virtual ground memory design can increase array density while maintaining process compatibility with existing semiconductor processes. Representative examples of the virtual ground memory arrays are disclosed in U.S. Pat. Nos. 3,916,169, 3,934,233, 4,021,781 and 4,387,447.
While virtual ground memory designs enjoy advantages from increased bit density, they have a number of disadvantages. One disadvantage is the problem of unwanted interaction between adjacent memory cells. This interference can take the form of a program disturb condition, in which programming of a selected cell leads to unwanted programming of unselected adjacent memory cells. This interference can also take the form of a read access degradation due to an unwanted current component. In both situations, the interference affects memory cells located in adjacent columns and connected to the selected row line. Interaction between adjacent cells also causes parasitic currents which interfere with reading, erasing and programming of individual cells. Ultimately, the access speed and integrity of the memory array is adversely affected by these problems.
What is needed is a programming mechanism for virtual ground EPROM memory arrays that allows for programming of a selected memory cell without disturbing data stored on adjacent unselected memory cells.